Nitrogen exists in different forms in the environment and living organisms (such as in amino acids, proteins and nucleic acids (DNA), as just a few examples). The increasing use of nitrogen-containing fertilizers today can lead to eutrophication in ground water, inland fresh water sources (including lakes) and even coastal sea water. This in turn can cause algae blooms, since nitrogen is one of the main nutrients for most plants and algae. Eutrophication will result in an over-consumption of oxygen in aquatic environments and disruption of the ecological balance.
Increasing nitrate concentration in water also causes harmful effects to human health. Methaemoglobinaemia (also known as “blue baby syndrome”), which causes patients to have a significantly compromised ability to carry vital oxygen in their blood, is mainly due to excess nitrate in drinking water. In the U.S., the Environmental Protection Association (EPA) regulates the maximum allowable contamination level for nitrate as nitrogen (NO3—N) to be 10 ppm (10 mg/L) so as to prevent adverse health effects on people. Nitrite (NO2−), a kind of reduced form of nitrate, is another major cause of mass fish kills in contaminated lakes and near seashores, and it is detrimental to human health as well. Therefore, effective and accurate detection of nitrate concentrations in water samples in the field has become significant and imperative.
Currently, the general public is not equipped to detect invisible contaminants such as nitrate in a water source because in general such detection requires specialized instrument and reagents, a certain level of expertise and, in some cases, high costs. In some special circumstances, it is strongly desired for people like environmental scientists, tourists and soldiers to have the ability to precisely determine nitrate contamination in environmental water sources they encounter. Therefore, an easy to operate, accurate, sensitive and portable platform for nitrate detection in the field is desirable.
Many existing methods to determine nitrate concentration are available, including spectroscopic methods, chromatography, capillary electrophoresis and electrochemistry methods. Spectroscopic methods are broadly used for nitrate determination, mainly using UV/Vis or fluorometric emission. These methods normally involve the use of chemical reactions like the Griess reaction to form highly fluorescent compounds to be compared and measured using a spectrophotometer. The detection limit achievable by these methods is in between 0.02 and 2 μM. But multiple required reaction steps make these methods time consuming. Also, the instrument for fluorescence spectroscopic detection is not readily accessible to non-scientists. Chromatography methods, either gas chromatography or ion chromatography, are commonly used in laboratory set-ups. Although the processes are relatively simple and fast in the laboratory, some requirements like high pressure for HPLC (high-pressure liquid chromatography) and a long column (200 mm) make it difficult to miniaturize these techniques. The flow rate control of the mobile phase is also sensitive to vibration or trembling and hence is unsuitable for a portable device. Further, due to the associated high costs and large form factor, it is unlikely to be adopted by everyday user for routine nitrate measurements. For capillary electrophoresis methods, the required column length is even longer and the set up for fluorescence detection is also complicated.
Compared with other methods, electrochemistry methods are relatively easy to operate and more readily transformable to a portable platform. In these electrochemical methods, application of potential between reference and working electrodes, causes oxidation or reduction of electro-active species to occur. Cyclic voltammetry is a popular method often used in electrochemistry systems. It would be helpful to apply electrochemistry methods in a portable platform approach to allow everyday users to quickly, cheaply and accurately record nitrate concentration and other similar environmental data in the field.